Diffusion barrier layers

ABSTRACT

A method of applying a stable intermetallic diffusion barrier layer to a metallic article is described. In many instances, protective coatings are incompatible with the material of the substrate to which they are applied. Such incompatibility is overcome in the invention through in situ formation of an intermediate stable diffusion barrier layer by sequential deposition and subsequent reaction of suitable metals to form a continuous intermetallic layer. A conventional overlay coating may then be applied to the intermetallic layer without risk to the underlying substrate. The invention also contemplates creation of unitary diffusion barriers from multi-layer deposits; deposition of plural diffusion barriers, and formation of complete protective systems comprising substrate, diffusion barrier(s) and overlay coating prior to heat treatment in situ.

This application is a 35 U.S.C. § 371 national phase application ofPCT/GB94/00301, filed Feb. 15, 1994.

The present invention relates to protective coatings for metallicarticles, and in particular to stable diffusion barrier coatings whichcan be applied to inhibit the deterioration of substrate materials as aresult of interaction with external stimuli.

BACKGROUND OF THE INVENTION

The utility of many materials is limited by their tolerance of operatingconditions since, in hostile environments such as extended exposure tohigh temperatures and temperature cycling of the type found in gasturbine engines, environmental degradation can be very severe.

Over the years, technology has developed a variety of protective ecoatings to extend the operating lifetime and/or the maximum permittedworking temperature of many materials. However, the coatings of choicefor particular applications, for example coatings having good oxidationor corrosion resistance, are not necessarily compatible with thesubstrate material to which they are applied. In many cases there areunfavourable interactions between the material of the substrate and thecoating composition, with the result that the physical and mechanicalproperties of the substrate are compromised. Deterioration of metallicmaterials is accelerated at elevated operating temperatures.

Formation of intermetallic protection layers of high temperaturecomponents is known in JP 570155364, where a PtAl₂ discontinuousintermetallic phase of approximately 35-50 μm is formed by a diffusionpack aluminising process at a temperature of 1150° C. High temperaturematerials are defined as those materials capable of operating attemperatures of 500° C. or greater. This protection layer does notafford uniform protection for the substrate material.

There is therefore a need for a uniform coating technique for hightemperature components which will allow the best possible coatings for agiven purpose to be applied to a particular substrate regardless of theinteractions which might otherwise occur between the two.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of the sequence of steps necessary tocarry out the invention.

DESCRIPTION OF THE INVENTION

According to the present invention, this is achieved by applying astable continuous uniform diffusion barrier as an intermediate coatingbetween the protective coating and the surface of the high temperaturesubstrate material. The diffusion barrier serves to inhibit thebreakdown of the protective coating system by minimisingcoating/substrate interactions, such that efficacy of the protectivecoating is maintained even though the composition of the coating may bealtered by loss through surface oxidation/corrosion. The diffusionbarrier also helps to preserve the physical and mechanical properties ofthe substrate, by limiting unfavourable interactions.

In particular, the inventive technique relies on the in situ formationof a continuous stable diffusion barrier by means of sequential layeringand subsequent reaction treatment of suitable metallic species toproduce a diffusion barrier of intermetallic form. This inventiveconcept extends also to diffusion barriers consisting ofmulti-intermetallic layers each limiting the diffusion of a specificelement (or elements), and is not necessarily limited to the formationof a single intermetallic diffusion barrier layer of homogeneousstructure. By selection of appropriate intermetallic species,interdiffusion of the protective coating through the barrier can beminimised.

The invention is a method of producing a continuous stable intermetallicdiffusion barrier on a metallic high temperature article, the methodcomprising the steps of:

depositing at least a first layer of a first metal on the surface of thearticle;

depositing at least a second layer of a second metal on the surface ofthe article to a depth sufficient to provide a predetermined molar ratioof the first and second metals, and

performing a reaction treatment which causes the first and second metalsto combine to form an intermetallic species.

The term metallic is used to define substrates made of metal,intermetallic or alloy materials.

For certain applications, it may be advantageous to use a number ofsequential deposition steps to build up the requisite thickness of firstand second metals prior to the reaction step.

As indicated above, it may sometimes be advantageous to deposit aplurality of intermetallic diffusion barriers, each of which serves as abarrier against diffusion by particular species.

It is also possible to build up a complete protective system comprisingthe metallic article substrate, diffusion barrier(s) and overlay coatingprior to heat treatment in situ of the complete protective system. Inthis condition, it is important to ensure that the top-most metal of thediffusion barrier precursor has a low interaction with the overlaycoating.

The reaction treatment step may be carried out prior to or during thenormal alloy heat treatment cycle to which the metallic article issubject. Preferably the reaction treatment is a simple heating stepwhich involves raising the deposited metals to a sufficiently hightemperature to initiate the exothermic reaction necessary to form theintermetallic species. This may be performed under moderate vacuum tominimise depletion of the second metal layer by atmospheric oxidation.

Alternatively, the reaction treatment could be carried out at highpressure, for example using a hot isostatic pressing technique. Apartfrom simple heating, the reaction treatment step could also be performedby thermally exciting the first and second metal layers using a laserbeam, plasma treatment, or any other high energy surface treatment.

Preferably the diffusion barrier layer formed by the method of thisinvention is of a thickness between 0.1 μm and 10 μm, although morepreferably the barrier layer thickness is between 0.8 μm and 3.0 μm.

The invention will now be described by way of example with reference toFIG. 1 which shows in schematic form the sequence of steps necessary tocarry out the invention.

Referring now to the Figure, step 1 represents the deposition of a firstlayer of a metal "A" upon the substrate. The thickness t_(A) of thisfirst layer is determined by the overall thickness required for thereacted thermal diffusion barrier coating and the stoichiometricproportion of metal "A" present within that coating.

Step 2 represents the deposition of a second metal "B" over the firstlayer deposited in step 1. Again, the thickness t_(B) of this secondlayer is a function of the required thickness for the finished thermaldiffusion barrier and the stoichiometric proportion of metal "B"present.

Step 3 represents the reaction treatment step which results incombination of the two discrete layers of metals "A" and "B" into asingle diffusion barrier layer of intermetallic "AB".

Assuming the reacted diffusion barrier layer A_(x) B_(y) is astoichiometric intermetallic product, in order to obtain the desiredstoichiometry it must contain proportionally x moles of metal "A" and ymoles of metal "B". If the respective atomic weights (M) of the twometals are M_(A) and M_(B) and their densities (p) are p_(A) and p_(B),then the ratio of thicknesses t_(A) and t_(B) should be in the sameratio as: ##EQU1##

This relationship assumes that thickness is directly proportional tovolume. It further assumes that little or no depletion of either layeroccurs due to solid/gas or solid/solid interactions. Where suchdepletion occurs to an appreciable degree, appropriate adjustments mustbe made to the relative thicknesses of the respective layer or layers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

The above technique has been successfully carried out on a substrate ofcommercially available IMI834 titanium alloy. In this case, a diffusionbarrier was required to serve as an intermediate layer between thesubstrate and an oxidation resistant coating. The purpose of theoxidation resistant coating was to inhibit the ingress of oxygen,thereby limiting the formation of a brittle α-case layer which wouldotherwise severely reduce the mechanical properties of the titaniumsubstrate.

The sequence of steps outlined above was employed to form anintermetallic PtAl₂ layer from the reaction of sequentially applied Ptand Al layers using an R.F. biased D.C. sputtering route.

In order to minimise potential solid/solid interactions between thesubstrate material and the first metal "A", particularly in view of thehigh diffusivity of aluminium in titanium, the relatively slow diffusingPt layer was deposited first. This was followed by deposition of therequired thickness of Al in accordance with the ratio: ##EQU2##(remembering that for PtAl₂, x=1 and y=2).

Depletion of the outer aluminium layer due to atmospheric oxidation wasprevented by carrying out the reaction treatment in a moderate vacuum ofroughly 2.0×10⁻⁵ bar. The reaction treatment in this instance consistedof heating for a period of 2 hours at a temperature of 750° C. X-raydiffraction analysis of the surface of the diffusion barrier coatingconfirmed that the desired PtAl₂ crystal morphology had been obtained.

The continuous PtAl₂ intermetallic layer thus formed was thenoverlay-coated with an 80/20 Ni/Cr oxidation resistant layer. Anoxidative heat treatment for 100 hours in air at 700° C. subsequentlydemonstrated the complete effectiveness of the PtAl₂ layer as adiffusion barrier for nickel. Moreover, etching of the substrate surfacerevealed no evidence of α-case formation, confirming the efficacy of theNi/Cr layer as a barrier to oxidation.

Other intermaetallics formed as diffusion barriers on IMI834 substrateare TiAl and (PtTi₃ +TiAl). Nickel based substrates have also beensubjected to the method of this invention, with each of NiCr, NiCrAl,NiAl and Ni having a diffusion barrier of PtAl₂.

Following diffusion barrier formation on the NiCR and NiAl substratesand application of an overlay coating system, each specimen wassubjected to oxidative heat treatment for 80 minutes at 1050° C. andalso for 40 minutes at 1150° C. The (PtTi₃ +TiAl) diffusion barrier on asubstrate of IMI834 was subjected to an oxidative heat treatment of 700°C. for 100 hours. Each of the treatments demonstrated the efficacy ofthe respective diffusion barrier layers.

Titanium aluminite alloys formed the substrate for intermetallicdiffusion barrier layers of PtAl₂ and also TiAl.

The table below summarizes typical diffusion barrier formation forparticular substrate materials, with associated typical thickness ofdiffusion barrier layers and also the efficacy testing conditions.

    ______________________________________                                                           Thickness range                                                                           Stable exposure                                Substrate                                                                            Diff. Barrier                                                                             (total) μm                                                                             conditions                                     ______________________________________                                        IMI 834                                                                              PtAl.sub.2  0.7-3.2     Up to 100 hrs at                                                              700° C.                                 IMI 834                                                                              TiAl        1.9           --                                           IMI 834                                                                              (PtTi.sub.3 + TiAl)                                                                       3.0         100 hrs at 700° C.                      α.sub.2                                                                        PtAl.sub.2  0.6-1.8                                                    α.sub.2                                                                        TiAl        1.9                                                        NiCr   PtAl.sub.2  1.8-4.0     80 mins/1050° C. &                                                     40 mins/1150° C.                        NiCrAl PtAl.sub.2  1.8-4.0                                                    NiAl   PtAl.sub.2  1.8-4.0     80 mins/1050° C. &                                                     40 mins/1150° C.                        Ni     PtAl.sub.2  1.8-4.0                                                    ______________________________________                                    

From the examples it can be seen that a number of diffusion barrierintermetallics can be used for differing substrates. It will beunderstood that alternative oxidation resistant overlay coatings couldhave been used. It will also be appreciated that suitable overlaycoatings need not be confined to those imparting oxidation resistance.

We claim:
 1. A method of producing a metallic high temperature article comprising a metallic substrate, a protective coating and a continuous uniform intermetallic diffusion barrier layer positioned between said substrate and said protective coating, said diffusion barrier layer comprising a first metal, A, said first metal having a molecular weight M_(A) and density p_(A) and a second metal, B, said second metal having a molecular weight M_(b) and density p_(b) in proportion x moles A to y moles B, the method comprising the steps of:(a) depositing at least a first layer of said first metal on the surface of the metallic substrate, said first layer being of thickness t_(A) ; (b) depositing at least a second layer of said second metal on the surface of said first metal, said second layer being of a thickness t_(B), the ratio of said thickness t_(A) :t_(B) being substantially equal to the ratio: ##EQU3## (c) reaction treating to a sufficiently high temperature to initiate an exothermic reaction thereby causing said first and second metals to combine to form a stable, continuous uniform intermetallic diffusion barrier layer formed of an intermetallic species; and (d) depositing said protective coating on the surface of said intermetallic diffusion barrier layer.
 2. A method as claimed in claim 1 wherein a plurality of sequential deposition steps is employed to build up the thickness t_(A) and t_(B) of said first and second metals prior to said reaction treatment.
 3. A method as claimed in claim 1 wherein said steps (a), (b), and (c) are repeated thereby depositing a plurality of sequential intermetallic diffusion barrier layers of differing compositions to serve as barriers against diffusion by a variety of species.
 4. A method as claimed in claim 1 wherein said reaction treatment comprises heating said deposited layers of first and second metals to a temperature sufficient to effect exothermic reaction between said first and second metals.
 5. A method as claimed in claim 1 wherein the reaction treatment comprises thermal excitation of said deposited layers of first and second metals by a high energy surface treatment.
 6. A method as claimed in claim 5 wherein said high energy surface treatment utilizes a laser beam.
 7. A method as claimed in claim 5 wherein said high energy surface treatment is a plasma treatment.
 8. A method as claimed in claim 1 further comprising applying an overlay coating to the outermost layer of the intermetallic diffusion barrier or barriers prior to said reaction treatment step to form part of a complete protection system comprising the metallic article substrate, intermetallic diffusion barrier layer or layers and overlay coating, and said reaction treatment comprises reaction treating said complete protection system in situ.
 9. A method as claimed in claim 1 wherein said reaction treatment step is carried out at non-ambient pressure.
 10. A metallic high temperature article comprising a metallic substrate, a protective coating and a continuous uniform intermetallic diffusion barrier layer positioned between said substrate and said protective coating, said diffusion barrier layer comprising a first metal, A, said first metal having a molecular weight M_(A) and density p_(A) and a second metal, B, said second metal having a molecular weight M_(B) and density p_(B) in proportion x moles A to y moles B produced by the method of claim 1 and the total thickness of the diffusion barrier layer is between 0.1 μm and 10 μm.
 11. A metallic high temperature article as claimed in claim 10 wherein the total thickness of said intermetallic diffusion barrier layer or layers is between 0.8 μm and 3.0 μm. 